This invention relates to a displacement sensor for optically measuring the distance to a target object. More particularly, this invention relates to such a displacement sensor adapted to be used together in combination with another displacement sensor to measure the step height or the thickness of a target object.
FIG. 12 shows a prior art displacement sensor 50 of a type comprised of a detector 60 and a signal processor 70. The detector 60 contains a light emitter 62 and a light receiver 62. The light emitter 62 includes a light source 64 such as a laser diode adapted to emit detection light f driven by a pulse from a driver circuit 63 and a light projecting lens 65 for focusing the detection light f to make incidence onto the target object 80. The light receiver 62 includes a light receiving lens 66 for collecting reflected detection light f from the target object 80 and a one-dimensional position detecting element 67 such as a PSD or a CCD for focusing the light collected by the light receiving lens 66. This position detecting element 67 is adapted to output a detection signal i according to the displacement of the focusing position from a reference position. Since the relationship between the distance to the target object 80 and this displacement is non-linear, the signal processor 70 carries out a correction calculation for correcting this non-linearity.
The signal processor 70 includes a sample-and-hold circuit 71 for receiving the detection signal i from the position detecting element 67 and an A/D converter 72 for converting the signal received from the sample-and-hold circuit 71 to a digital signal. The converted output signal from the A/D converter 72 is received by the CPU 73 of a microcomputer. The CPU 73 carries out processes such as the non-linearity correction referred to above and calculates the distance D to the target object 80. The calculated distance value d is outputted as an analog signal through a D/A converter 74 and an output circuit 75. The CPU 73 also compares the calculated distance value d with a specified threshold value, generates a judgment signal on the basis of this comparison and outputs this judgment signal.
With a displacement sensor thus structured, the distance to a target object can be easily measured, but it cannot easily measure by itself the step height or the thickness of an object. FIG. 13 shows an apparatus combining two displacement sensors 50A and 50B with a calculation controller 51 capable of measuring the step height or the thickness of a target object. For this purpose, the detectors 60A and 60B of the displacement sensors 50A and 50B are positioned at a same height above the target object 80. The detector 60A of the first displacement sensor 50A is positioned above the higher part 81 of the step H and emits detection light thereonto to receive detection light iA. The detector 60B of the second displacement sensor 50B is positioned above the lower part 82 of the step H and emits detection light thereonto to receive detection light iB. Signal processors 70A and 70B respectively of the first and second displacement sensors 50A and 50B calculate the distances D1 and D2 to the higher and lower parts 81 and 82 of the step H. The calculated data d1 and d2 for the distances D1 and D2 are received by the calculation controller 51 which calculates their difference as the step height H and outputs its calculated value h.
FIG. 14 shows the apparatus shown in FIG. 13 being used to measure the thickness of a target object (also indicated by numeral 80). The detectors 60A and 60B of the first and second displacement sensors 50A and 50B are positioned so as to face mutually oppositely, sandwiching the target object 80 in between. The detector 60A of the first displacement sensor 50A projects light to one surface of the target object 80 and the detector 60B of the second displacement sensor 50B projects light to the other surface of the target object 80 to obtain detection signals iA and iB. The signal processors (shown at 70A and 70B in FIG. 13) of the displacement sensors 50A and 50B calculate the distances D1 and D2 to the corresponding surfaces of the target object 80. The calculated data d1 and d2 for these distances D1 and D2 are received by the calculation controller (shown at 51 in FIG. 13) and calculate the thickness T of the target object 80 by subtracting the sum of d1 and d2 from the known distance K between the detectors 60A and 60B. The calculated data for the thickness T is then outputted.
FIG. 15 shows another apparatus capable of measuring the step height and the thickness of a target object as well as the distance thereto, comprising a pair of detectors 60A and 60B and a signal processor 90. This signal processor 90 includes signal processing circuits 91a and 91b which correspond to the signal processors 70A and 70B described above with reference to FIG. 13 and a calculation circuit 92 which corresponds to the calculation controller 51 described above with reference to FIG. 13.
When the step height H of a target object 80 is measured by this apparatus, the signal processing circuits 91a and 91b process the detection signals iA and iB from the detectors 60A and 60B to calculate the distances D1 and D2 to the higher and lower parts 81 and 82 of the step H, and the calculating circuit 92 calculates the difference between the distance data d1 and d2 for the distances D1 and D2 and outputs the difference h.
With apparatus shown in FIGS. 13 and 14, not only the distance to a target object but also step height and thickness of the target object can be measured. On the other hand, since two displacement sensors 50A and 50B and a calculation controller 51 are required, the apparatus is expensive and requires a relatively large space for installation.
The apparatus shown in FIG. 15 is advantageous in that the single signal processor 90 is required for providing the function of receiving detection signals from the pair of detectors 60A and 60B and calculating a distance and the function of calculating the step height and the thickness from two calculated distance data and hence that a large space is not required for its installation. For a user interested only in measuring the distance to a target object, however, the apparatus is unnecessarily expensive, two detectors being not required and the signal processor 90 being provided with unnecessary functions.